Many machines, including large off-highway machines, are known to use hydrostatic drive systems to drive the ground engaging elements, such as wheels or tracks, of the machine. Such hydrostatic drive system commonly includes at least one pump driven by a prime mover, such as an internal combustion engine, of the machine. The pump may be configured to drive one or more sets of motors, which, in turn, power the ground engaging elements of the machine. The pump, and/or motors, may provide variable displacement, such that a fluid flow between the components of the hydrostatic drive system may be adjusted while the machine is running. As a result, direction, speed, and torque of the drive wheels may be continuously varied.
These machines typically include a service brake, or other wheel brake, for slowing or stopping the machine. Utilizing such brakes, however, may cause wear and/or damage to components of the drive system and, additionally, may add weight and cost to the machine. Alternatively, U.S. Pat. No. 5,111,658 teaches a method of braking a machine by controlling a pump of a hydrostatic drive system. Specifically, an electronic controller rapidly adjusts the displacement of the pump between one setting that permits the drive motor, or engine, to exert a braking moment and another setting that relieves the drive motor after a limiting speed value has been exceeded. The latter setting may incorporate the use of pressure limiting valves to reduce pressure within the hydrostatic drive system and, as a result, reduce the speed of the drive motor. This rapid adjustment between settings may occur until the machine is sufficiently slowed or stopped.
While the cited braking method may provide adequate slowing or stopping of the machine, it should be appreciated that there remains a continuing need for improved braking strategies and methods for hydrostatic drive machines. Specifically, there is a need for braking strategies having improved efficiency that may reduce the speed of the machine, such as according to requisite standards, reduce overspeeding of the components of the hydrostatic drive system, and reduce the need for a service brake, or wheel brake.